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Authors: Dr. Shah Aakash M, Dr. Shah Purvesh M, Dr. Shah Romil B

Abstract:

When patients think about correction of malaligned teeth, braces and wires are the first thing that spring to mind. However, orthodontics has revolutionized according to the demands and needs of the patients. Orthodontists are concerned about the aesthetics and it is one of the major concerns among patients who seek orthodontic treatment. To address the increasing aesthetic demand for an alternative to conventional braces, investigators have developed several solutions, such as ceramic or composite braces, lingual orthodontics, and clear aligners. Invisaligners are transparent, thin, and custom made, removable plastic aligners to move the teeth into the desired position. Invisalign was introduced in the late 1990s by Align Technology Inc, and due to advanced technology allowed a much simpler approach to this type of treatment.

Keywords: Invisalign, Clear aligners, Orthodontics, Malocclusion

Introduction

In recent years the demand for aesthetic orthodontic appliances has increased dramatically. Consequently, sequential clear thermoplastic aligners have become a popular alternative to fixed appliances. The concept of aligning teeth with thermoplastic appliances is not new. The use of a flexible removable orthodontic appliance for minor tooth movement was first introduced by Kesling in 1945. 1 The “tooth positioning appliance” was initially made from rubber and was a one-piece flexible appliance that covered the surfaces of the upper and lower teeth. It allowed active tooth movement -and was indicated for the treatment of mild relapse and for use as a retainer. 1

With the advent of vacuum-formed clear thermoplastic sheets it became apparent that if teeth were reset slightly and the vacuum-formed sheet was made to fit the reset teeth, a tooth moving device would be the result. Nahoum described his “vacuum formed dental contour appliance” in 1964 and was one of the first to apply elastics and utilise attachments. 2 Ponitz in 1971 introduced the concept of the “invisible retainer” and acknowledged that these thin thermoplastic appliances could be used to move teeth. 3 McNamara also discussed the use of invisible retainers for minor tooth movement. 4 Such devices became known as “aligners” because the typical use was to bring mildly misplaced teeth back into alignment. 5 The early appliances were manufactured with vacuum-form machines that sucked the heat-softened thermoplastic material onto the model. The vacuum method was found to have inaccuracies in areas where the vacuum pressure was unable to reach effectively. 6 Within the last decade machines which use compressed air to blow the material onto the cast have improved the accuracy. 7 Commercially available pressure machines include Biostar, Erkopress and Trutain. 6

Only small amounts of tooth movement are possible with a single aligner because of the stiffness of the plastic material. To obtain more than minor changes, it is necessary either to reshape the aligner or make a new one on a cast with the teeth reset to a greater degree. A sequence of several aligners made on a series of casts with reset teeth, each incorporating another small amount of tooth movement, is referred to as clear aligner therapy or clear sequential aligner treatment. Different systems have evolved to facilitate a broader range of tooth movement with clear aligner therapy.5

Essix developed a technique whereby clear pressure-formed thermoplastic appliances are used to perform minor tooth movements and this became known as the “Essix Appliance”.8 In the early 1990s Sheridan popularised the Essix appliance.9, 10 The appliances are constructed from unaltered plaster casts of the patient’s teeth and tooth movement is achieved by placement of a divot which applies pressure to the tooth when the appliance is in place. Space within the appliance is obtained by blocking out the working cast or cutting a window in the appliance. The movements are limited to a maximum of 3mm (in 1mm increments) as the plastic becomes too thin to exert force after this. 5 This system avoided the cost and complexity of having to make multiple new aligners. 9 Despite the improvements, reshaped aligners are not considered a practical way to manage orthodontic problems of any complexity. 5

Commercially available systems

Three commercial systems involving the use of a series of clear thermoplastic appliances for sequential tooth movement in the treatment of malocclusions have been available: ClearSmile, Simply 5 and Invisalign®. ClearSmile Pty Ltd was formed by a team of orthodontists and technicians in NSW. 11 In this system thermoplastic appliances known as “correctors” are used to treat malocclusions. From a single polyvinyl-siloxane (PVS) impression, a technician manually resets teeth in sequential stages on the plaster model and fabricates a series of correctors. Each appliance is designed to move the teeth in approximately 0.5mm increments. 12 ClearSmile list the applicability of their appliances as follows: Class I molar relationship, crowding less than 4mm, spacing less than 5mm, overjet less than 5mm and openbite of less than 1mm. Although a sequence of modified dental casts can be produced manually in a standard dental laboratory, this is time consuming and difficult. 5 The company has recently ceased operations. Another commercially available aligner system is Simpli 5TM manufactured by AOA Orthodontic Laboratory, Inc and marketed by Ormco Pty Ltd.13 It is designed to treat patients with mild to moderate anterior crowding or spacing, or those who have experienced orthodontic relapse and have a stable posterior occlusion and no TMD. It is a laboratory generated product that delivers five sets of sequential trays for anterior correction that require up to 2.5 mm of movement per arch from impressions or models. The reported advantages of Simpli5 include speed, flexibility, simplicity and economy.13

Invisalign

The Invisalign® System was introduced by Align Technology Inc (Santa Clara, California) in 1999. In this system a series of clear, removable, plastic appliances that are worn sequentially by a patient are used to correct a malocclusion.7, 14 Align Technology Inc computerised the process of producing sequential aligners. Traditional laboratory methods are labour intensive and require detailed setups to be done by a technician. Consequently, this technique is used for simple malocclusions and is difficult to apply to a large patient population. By developing a computer-based manufacturing process Align Technology was able to resolve some of the difficulties.14, 15

The Invisalign® System requires a CT scan of a PVS impression creating a digital model and uses 3- D computer software to manipulate the position of the teeth on the digital model. Sequential stereo- lithographic resin models are created with a computer-programmed laser. From these models, a series of vacuum-formed appliances known as “aligners” are constructed. The movement programmed into each aligner is 0.25 to 0.33mm.16, 17 The aligners are worn for a minimum of 20 hours per day and changed every two weeks.7 Since its advent Invisalign® has grown rapidly in worldwide consumer demand and professional use. Currently over one million patients have been treated with Invisalign.18 Meier et al. conducted a prospective study to define a profile of patients who were interested in Invisalign®. They found women aged between 20 and 29 years were most frequently interested in Invisalign treatment. 97% of those surveyed gave aesthetic concerns as their primary motivation for treatment. The demand for aesthetic treatment options were also reflected in the finding that 62% would not consider orthodontic treatment with visible appliances.19 Vicens and Russo recently investigated the use of Invisalign by orthodontist and general dentists within a 35-mile radius of Stony Brook University. Interestingly, for both groups, the longer the practitioners were certified in Invisalign, the fewer cases they started over the last 12 months. The authors suggested that for these practitioners the novelty of the technique had diminished and that its limitations relative to fixed appliance treatment are beginning to discourage them from using it as much as they originally did. 20 Indications

Align Technology provides guidelines for cases that can be successfully treated with Invisalign. Cases for which Invisalign is recommended include the following features: Mild to moderate crowding (1- 6mm), mild to moderate spacing (1-6mm), non-skeletal constricted arches and relapse after fixed appliances. The case selection criteria of Align Technology are merely guidelines. Each clinician must apply their only clinical judgement regarding the suitability of the case, as they are responsible for the treatment outcome. 21

Several case reports have documented successful treatment of mild to moderate malocclusions with the Invisalign® system. 22-24 Boyd et al. published the first case reports of treatment with the Invisalign system in 2000. The first cases treated with Invisalign were adult patients with mild (3 to 6mm) spacing and crowding. 23 Early studies demonstrated limitations in the treatment of complex cases with the Invisalign system. During the first four years of appliance development significant problems were accounted with accomplishing bodily movements, root torque, extrusion and derotation of canines and premolars.25, 26

Controversy still exists over whether moderate to difficult orthodontic treatment can be completed routinely with Invisalign.25 In recent times case reports of successful management of moderate to difficult malocclusions with Invisalign have appeared in the literature.27 Patients with more complex malocclusions including premolar extractions, deep overbites, Class II malocclusions, molar distalisation and open bites have been treated with the Invisalign system.25, 27-33 Despite this, a recent survey reported that most orthodontists and general practitioners would not treat severe Class I malocclusions with Invisalign.20

The inability to control root movement limits the use of the Invisalign system in malocclusions requiring premolar extractions. 34 This is considered to be one of the most significant limitations of the appliance.35 Case reports by Giancotti et al. and Miller et al. which involved premolar extractions highlight this problem, as both required fixed appliances to upright the molars, premolars and canines at the completion of aligner therapy.34, 36 Honn and Goz presented a case report of a successful premolar extraction treatment with Invisalign. One aspect favouring the use of the system was that limited bodily movement was required, only minor rotations and no extrusion, intrusion or torque movements. The author’s highlighted that the success of Invisalign treatment is largely dependent on which tooth movements are required to correct the clinical situation and the importance of understanding the range of indications for the appliance.29

Based on clinical experience Joffe advised that the Invisalign system has difficulty treating the following cases:

  • Crowding or spacing over 5mm
  • Sagittal discrepancies more than 2mm from a Class I canine relationship
  • Large discrepancies between centric relation and centric occlusion
  • Teeth that are rotated more than 20 degrees
  • Open bites
  • Cases requiring extrusion
  • Teeth that are tipped more than 45 degrees
  • Short clinical crowns
  • Multiple missing teeth.37

Although certain aspects of a malocclusion are difficult to manage with Invisalign, it does not preclude the use of the system completely, as it is possible to undertake combined treatment. It can be used to treat one arch or alternately it can be used in a staged treatment with fixed appliances. [35] Invisalign has also been used sequentially with a functional appliance and a Carriere distalising appliance. 38, 39

Efficacy of clear aligner therapy

Given that clear sequential aligner systems are fundamentally similar, the systems will be considered collectively with regard to the advantages, disadvantages, efficacy and tooth movements that can be achieved. Certain tooth movements are performed more predictably than others with clear aligner therapy. 32

As the demand and professional use of clear sequential aligners continues to grow the efficacy of the system needs to be examined. Adequate assessment of the effectiveness of Invisalign treatment is difficult as insufficient clinical research has been published.21 Lagravere and Flores-Mir performed a systematic review of the literature regarding the Invisalign system and found that scientific evidence regarding the indications, efficacy, limitations and treatment effects were lacking. Two articles fulfilled the inclusion criteria of a clinical trial but the authors determined that they did not adequately evaluate the treatment effects of the system.40 The majority of articles in the literature are case reports, material studies, commentaries and descriptions of the use of the system.15, 21 The authors reported that no strong conclusions regarding the treatment effects of Invisalign appliances could be made.4

The two clinical trials by Bollen et al. and Clements et al used different aligner material to Invisalign’s current system, which minimises their importance. 41 The first study by Bollen et al. investigated the effects of activation time and material stiffness on the patient‟s ability to complete Invisalign treatment. 41 The results of the study supported the current recommendation for a 14 day wear period as a 2-week activation time almost doubled the likelihood of successful completion of the aligners, compared to the 1-week activation. High PAR scores and planned extractions significantly decreased the likelihood that the aligners would be completed. Clements et al conducted the second clinical trial which examined the effects that activation time and material stiffness had on the quality of the dental movements as measured by changes in the PAR scores. 26 The authors concluded that the “aligners were most successful in improving anterior alignment, moderately successful at improving the overjet and midline, and least successful in improving buccal occlusion, transverse relationships, and overbite”. Analysis by extraction pattern revealed that incisor extraction sites had a significantly greater percentage of closure then premolar extraction sites.26 However, Invisalign appliances are now manufactured using a material of intermediate stiffness. 27

Djeu, Shelton and Maganzini compared the treatment outcomes of Invisalign cases and fixed appliance using the American Board of Orthodontics objective grading system.42 The overall passing rate for the Invisalign group was 27% lower than braces. Invisalign treatment finished 4 months sooner than fixed appliances. Invisalign was considered to be “especially deficient in its ability to correct large anterioposterior discrepancies and occlusal contacts”. The Invisalign system compared well to fixed appliances in regard to its ability to close spaces and correct anterior rotations and marginal ridge heights. A limitation of the study was the difference in the clinician‟s experience with the two treatment modalities. 43 The provider had less experience with the Invisalign system and refinements have been made to the technique since the cases were completed. 42 The same sample from the Djeu et al. outcome study was used by Kuncio et al. to compare the post-retention dental changes of patients using the ABO objective grading system. The authors found that the patients treated with Invisalign had more relapse, particularly in the maxillary anterior teeth. 44

There is considered to be a “lack of substantive controlled clinical trials” in regard to this treatment modality. 21 Further clinical trials are required to evaluate the strengths and limitations of Invisalign treatment. 21 Until better quality evidence is available, clinicians will have to rely on their clinical experience, the opinions of experts and the limited published evidence when using Invisalign appliances. 40

Tooth movements

There is limited published information about the force levels produced for tooth movement by the Invisalign system and other systems of this kind. 45 Duong and Kuo compared the force-strain characteristics of orthodontic wires (0.017x0.017 stainless steel and nitinol) to aligners and reported a lower level of strain for aligners, 1-2% strain, compared to stainless steel wires, which deliver an average strain of 4% when activated. No information was provided regarding how this data was obtained. 46 Barbagallo and co-workers used a novel pressure film approach to determine the force generated by clear thermoplastic aligners made from 0.8mm Erkudor thermoplastic blanks that had 0.5mm of buccal movement programmed in each appliance. Digital imaging and spectrophotometry analysis were used to quantify the strain intensity mounted by the pressure on the films. The results indicated that high force levels (5.12N) were applied to the tooth initially but diminished rapidly over the 2 week period of wear (-2.67N).

Tooth movements that are performed well with clear aligner therapy.5

  • Tipping
  • Rotation of incisors [25]
  • Intrusion (1-2 teeth) [5, 47]
  • Expansion
  • Constriction

Tooth movements that are not performed well with clear aligner therapy.5

  • Extrusion 37: This is considered to be one of the most difficult movements to achieve. Attachments are required to facilitate movement by creating an undercut area.32
  • Bodily movement during extraction space closure: this is primarily because the system has a limited ability to keep teeth upright during space closure.37
  • Torque (labiolingual tip)
  • Severe rotations (more than 20 degrees), especially premolars and canines. A survey by Sheridan revealed that “uncorrected rotations” were one of the most prevalent problems encountered by orthodontists using Invisalign, often resulting in the need for refinement or fixed appliance.48
  • Mesiodistal Tip – (Tipping) more than 45 degrees 37 Certain movements are possible using attachments:
  • Closure of premolar extraction space
  • Translation of molars
  • Extrusion of incisors

Efficacy of tooth movements with clear aligner therapy can by evaluated by comparing the planned virtual treatment with the actual treatment outcome. This information can help improve the appliance, guide future treatment decisions and clarify treatment indications. Align Technology has a software tool that can be used to superimpose digital models to evaluate treatment outcomes in three dimensions.14 Miller, Kuo and Choi showed that superimposition of digital models on the palatal rugae were reproducible and had a level of error similar or less than 2D cephalometric analyses. A single bicuspid extraction case was evaluated which showed that not all planned movements occurred. Most notably the treatment outcome showed that multiple teeth tipped into the extraction site.36

Kravitz et al. evaluated the efficacy of different tooth movements with Invisalign. 49 The amount of tooth movement predicted was compared with the amount achieved, using ToothMeasure, Invisalign’s proprietary superimposition software. The types of movements studied were expansion, constriction, intrusion, extrusion, mesiodistal tip, labiolingual tip and rotation. The mean accuracy of tooth movement with Invisalign was 41%. The most accurate movement was lingual constriction (47.1%) and the least accurate movement was extrusion (29.6%), especially for maxillary and mandibular central incisors. The findings of this study are likely to vary from clinical setting as the research protocol prevented the use of auxiliaries and did not account for overcorrection. 49 These results were less than the internal test results of Nguyen and Cheng who found a mean accuracy of 56% for anterior tooth movement.50 In addition, the internal study by Nguyen and Cheng revealed that the overall accuracy of canine and premolar rotation was only 39%.

To overcome some of the limitations of the appliance resin attachments are placed on the teeth.7, 32 In most circumstances the attachments increase the undercuts and retention of the appliance to facilitate the desired tooth movement. There are three fundamental types of attachments: those that assist tooth movement, those that augment retention of the appliance and those that assist auxiliary functions. All three categories of attachments act as force transmitters.7

Attachments vary in size and shape. The standard Invisalign attachment shapes are ellipsoid and rectangular. The dimensions of the ellipsoid attachments are height 3mm; width 2mm; and prominence of 0.75mm. The dimensions of the rectangular attachments can vary with heights of 3, 4 and 5mm; width 2mm and prominence of 0.5 or 1mm. They can be requested in horizontal or vertical orientations and with bevelled edges. 7

Invisalign has introduced new optimised attachments for extrusion, rotations and torque (power ridges). These attachments are automated and pre-activated. According to Align Technology the optimised attachments cannot be moved, lengthened or repositioned as they are customised for each tooth and are based on biomechanical studies.18 There is limited clinical information about the effectiveness of the new attachments at present.

The attachments are used for increasing aligner retention and tooth control. Attachments are formed by bonding tooth coloured restorative material to the buccal surfaces of the teeth and give the aligners‟ greater rotation and angulation control. [37] According to the experiences of Joffe, although attachments give the aligners greater rotation and angulation control it is only partially effective. He also acknowledges that as materials improve attachments will allow much greater control over tooth movement. 37

Kravitz et al. performed a prospective clinical study to evaluate the influence of attachments and interproximal reductions (IPR) on the accuracy of canine rotation with Invisalign. 51 53 canines were examined and the mean accuracy of rotation with Invisalign was found to be 35.8%. These results agreed with findings of Nguyen and Cheng regarding the difficulty derotation canines and premolars.7 There was no statistical difference in rotational accuracy among the groups – attachment only, IPR only or neither. The highest accuracy was achieved when IPR was performed. The author’s acknowledged the limitations of the study which included small sample size, lack of evaluation of IPR and failure to consider overcorrection. Further clinical tests were recommended regarding the placement and shape of Invisalign attachments, staging and amount of IPR, amount of overcorrection, and speed of tooth movement to improve the accuracy of rotating teeth. 51

Other authors have also commented that auxiliaries such as elastic and detailing pliers are required to facilitate tooth movement with clear aligners. Align Technology recommends interproximal reduction, thermopliers, overcorrection and axillaries in addition to attachments to aid rotational movements. 16 Boyd recommends 10% overcorrection whereas Kuo suggests 5% beyond the ideal and use of thermopliers when needed. 7, 51 Kravitz et al. recommended far greater overcorrection. 51

There are several factors that can affect treatment outcome with Invisalign. According to Duong and Kuo variation in biological response and tooth shape, such as irregular facial surfaces, unusual crown shapes and unfavourable crown shapes such as round teeth, reportedly affect the ability to achieve the desired outcome.46 Compliance is a considerable factor given this system is removable and compliance indicators have been recently added to Invisalign products.18 Different procedures have been recommended to improve treatment outcomes such as case refinements and detailing pliers.

Advantages

Numerous authors have given mention to the advantages and disadvantages of clear aligner therapy. [23] Reported advantages of clear sequential aligner therapy over conventional appliances are:

  • Excellent aesthetics 37
  • Facilitate good oral hygiene26
  • Ease of use for patients37, 52
  • More comfortable than fixed appliances 52
  • Ability to remove aligners to eat 44
  • Minimal need for adjustment
  • Reduced chair time23
  • Minimal impact on speech52
  • Potentially less root resorption45

Miller et al. conducted a prospective, longitudinal cohort study to compare the treatment impacts between Invisalign aligner and fixed appliance therapy during the first week of treatment in adult patients (33 with aligners, 27 with fixed appliances). [52] The Invisalign group experienced fewer negative impacts on their lives in relation to function, psychosocial impact and pain-related criteria. The visual analog scale pain reports demonstrated that adults treated with Invisalign experienced less pain and they also took less pain medication. The results of this study support the claims that Invisalign therapy is more comfortable and has a more favourable impact on patient quality of life compared to fixed appliances in the first week of treatment.52

Periodontal health benefits and improved oral hygiene have been cited as advantages of aligner therapy.32 Case reports of successful treatment in periodontally compromised patients have been documented in the literature to support such claims.32 Miethke and Vogt compared the periodontal health of patients during treatment with Invisalign and fixed appliances. 53 Thirty consecutive patients for each treatment modality were enrolled and the study evaluated the modified gingival index, modified plaque index, modified Papillary bleeding index and sulcus probing depth. The plaque index was found to be lower for the Invisalign group overall but the periodontal condition of the two groups was nearly identical. 53 A similar comparison was performed for fixed lingual appliances and Invisalign. 54 Clements et al reported a statistically significant decrease in average papillary bleeding score during treatment with aligners. They concluded that “unlike treatment with fixed appliances treatment with clear, removable aligners appears to have no adverse effects on gingival health during treatment”.26

A longitudinal study by Boyd suggested that patients with short roots may be “better candidates” for clear aligners than for fixed appliances. 27 Barbagallo et al investigated the amount of OIIRR generated by invisible removable thermoplastic appliances (ClearSmile) with a rate of tooth movement of 0.5mm every 2 weeks and light (25g) and heavy (225g) orthodontic forces. Over a treatment duration of 8 weeks it was found that thermoplastic appliances have similar effect on root cementum as light orthodontic forces with fixed appliances.55 These results agree with the few studies that show that removable appliances induce less OIIRR than fixed appliances. Brezniak and Wasserstein documented a case which had experienced orthodontically induced inflammatory root resorption of the four maxillary central incisors following Invisalign treatment. The authors intention was to demonstrate that this phenomenon can unpredictably appear with the Invisalign system, just as it does with all other orthodontic treatment modalities. “Force application, even by the Invisalign technique, initiates sequential cellular processes, as do all other orthodontic appliances that might lead to root resorption”. The author’s hoped the preference of the Invisalign system versus another treatment modality will not be related to the OIIRR phenomenon, because it can result from all treatment procedures.45

Computer-assisted processes, such as the Invisalign System, have additional benefits. Clinicians can evaluate multiple treatment options before finalising a treatment plan. The virtual treatment model can assist with patient communication and can serve as a motivational tool.14

Disadvantages
  • Short range of action
  • Poor three dimensional control of tooth movement
  • Limited effectiveness with other types of movements such as bodily movements, rotations, extrusions and severe intrusion of teeth Cannot control the angulation of a tooth when they are being moved
  • Compliance dependent23
  • Possible loss of appliance23

The current virtual dental models used in computer-assisted treatment planning and manufacturing are still considered to be incomplete.

14

The addition of root geometry could enhance the model. This information could be added by measuring root dimensions from radiographs or CT scans. The current gingival model is also incomplete. Shape changes are only approximations and the model does not account for extreme movements that can have detrimental effects, such as recession. 14

 

Conclusion

Flexible removable appliances are evolving rapidly. At present, the use of aesthetic removable appliances have not been shown to be as efficient as fixed appliances in the treatment of malocclusions, especially more complex cases. The appliance is dependent on patient compliance being a removable appliance. New compliance detectors are intended to overcome this disadvantage. In the future these appliances may become as efficient as fixed appliances as technology evolves.

Educating patients on the advantages and disadvantages of clear aligner therapy or clear braces significantly depends on patient’s expectations and compliance. First, as a provider, orthodontist must rule out conventional braces by having a clear communication with the patient. If the patient desires no treatment responsibilities, is compliant in visiting the office monthly, and wishes to have all of the treatment performed by the dentist, then conventional braces is the only treatment to recommend. Clear aligner therapy can be ruled out immediately. However, if patients desire the benefits of clear aligner therapy, the pros and cons must be presented to them. First, the patient must understand their compliance and responsibilities. They need to consistently wear the aligners 22 to 23 hours per day and only remove them to eat. One of the benefits of the aligner systems is the opportunity to see the end result of straight teeth and the progression of tooth movement during the multitude of stages. It is essential to continuously motivate each patient during treatment to properly wear aligners to avail benefits of the treatment, ensure patient compliance as well as patient self discipline.

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